1. Field of the Invention
The present invention relates to disc brakes and more particularly to improvements in large area contact disc brakes for large vehicles such as trucks, tractor-trailer vehicles in addition to other large wheeled vehicles and aircrafts.
2. Description of the Prior Art
Canadian Patents 1,112,189, issued Nov. 10, 1981, and 1,140,486, issued Feb. 1, 1983, U.S. Pat. No. 4,102,438, issued Jul. 25, 1978, Yvon Rancourt, and U.S. Pat. No. 5,205,380, issued Apr. 27, 1993, which are incorporated herein by reference, describe a disc brake for heavy road vehicles wherein the brake shoes are in contact with the much larger disc area, and a suitable brake cooling system is provided to cool the disc, thus making disc brakes practical for such vehicles. The present invention is an improvement over the above-mentioned patent.
There are braking systems available on various types of vehicles which include a positive mechanical brake of the type known as a fail-safe brake, that is, where the brakes are applied when pressure is released from the brakes. U.S. Pat. No. 3,547,234, Cummins, Dec. 15, 1970, describes a service brake for earth-moving trucks or tractors which includes a hydraulic brake system using a plurality of discs, and these discs are mechanically preloaded by a spring to provide a fail-safe brake. U.S. Pat. No. 4,057,297, Nov. 8, 1977, Beck et al, includes a brake system which has been preloaded by means of a spring, including the discs of the type described in the Cummins Patent. This pressurized air operated system uses a series of valves to modify the pressure exerted on the torque converter in order to avoid damage to the differential. This is a system that is utilized in very heavy machinery such as tractors, etc. These patents represent the state of the art in terms of fail-safe type brake systems using preloaded mechanical devices such as springs. The structures are complicated by the need to be adapted on heavy vehicles. What is required is a fail-safe type brake system of simple construction using pneumatic pressure for releasing the brakes and utilizing a simple disc brake construction of the type described in the Rancourt Patents wherein the disc is mounted to the adapter sleeve of a wheel on the vehicle and the brake housing is mounted to the vehicle on a dead axle.
It is also recognized that a major problem with large contact-area annular disc brakes of the type described in the above patents is heat. Great strides in improving heat dissipation were achieved with the introduction of vented discs as described in U.S. Pat. No. 4,102,438. However, vented discs of the type described required a thicker disc to retain the same strength. It was found for instance that it was not possible to house more than one cast-iron vented disc in an in-wheel brake housing, thus reducing the flexibility of design of such brakes, especially where multiple discs might be an advantage.
Another problem which has had a serious social impact is brake failure due to wear. Presently, there is no known reliable brake wear gauge for determining the remaining life of a particular set of brake linings on a truck vehicle. It is necessary to remove housing parts on the wheel in order to examine and measure the remaining thickness of the lining and the brake disc. Since such examination adds to the down time of the truck or tractor-trailer, the tendency of the operator or driver-operator is to delay such inspection, with sometimes disastrous results, often costing innocent lives in highway accidents due to failed brakes of such large vehicles.